They might be full of fruit and contain vitamins, but a UK study has found that most fruit smoothies contain more calories than Coca Cola.

The study was conducted by Which? –– similar to Australia's Choice organisation –– and found almost half of the 52 products tested had more than 30g of sugar per 250ml, which is the equivalent of six teaspoons of sugar.

Every smoothie tested had more calories than a 250ml bottle of Coke.

While the brands tested in the UK study were different to what's on offer in Australia, ninemsn did a calorie comparison and found that a lot of Australia's smoothies from juice bars and supermarket fridges also contain more calories than Coke.

A 250ml Nudie Orange, Mango and Pineapple Crushie has 457.7 kilojoules and 250ml of a Boost Juice Tropical Storm smoothie contains 675 kilojoules, compared with 450 kilojoules for a 250ml of Coke.

Dr Catherine Itsiopoulos, a spokesperson for the Australian Dietitians Association, told ninemsn that smoothies and juices tend to be calorie-rich and often aren't very filling.

"You could squeeze two or three pieces of fruit into a smoothie but you are not consuming the full benefit of eating two or three pieces of fruit – you're not getting fibre or as much vitamin C," she said.

"A liquid drink is often not sensed by your stomach as a food so you'll drink it as a drink and you'll still eat something else."

However Dr Itsiopoulos said you would get more nutrients from a smoothie than you would from Coke.

"Coke is only going to provide sugar and no other nutrients, whereas a smoothie does provide calcium, vitamin C, there also might be some protein and antioxidants," she said.

Smoothie-lovers are best to make them fresh to control the ingredients.

"You could use low-fat dairy products and use fresh fruit so the nutrient content is likely to be higher," Dr Itsiopoulos said. "It's within your control whether you add sugar or not."

It's also important to watch your portions.

"A 200ml glass is not going to provide calories in excess, whereas if you have a 500ml smoothie, then it is going to be more than double the calories," Dr Itsiopoulous pointed out.

"And don't tick off your fruit for a day –– make sure you have some fresh fruit through the day, too."

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